This application claims the priority of German application 197 55 044.4, filed Dec. 11, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a vehicle steering system, having a steering handle operated by the driver, particularly a manual steering wheel, as well as a motor-driven adjusting assembly which is assigned to the steered vehicle wheels and with which the steering handle is connected with respect to the effect by way of a control system whose controlling device arrangement is connected on the input side to a desired value generator for a steering angle to be set operated by the steering handle as well as to an actual value generator which records the steering angle of the vehicle wheels. By way of the system's output, the adjusting assembly is controlled as a function of a comparison of the desired and the actual value, as well as having a manual force adjuster connected with respect to the drive with the steering handle. The manual force adjuster simulates a parameter-dependent steering resistance on the handle. The steering handle and a servo motor provided as the manual force adjuster are connected with respect to the drive by way of a shaft. A controlling device controls the servo motor as a function of a comparison of the actual value signal and a signal of a desired-value generator.
Current passenger cars and the like are as a rule equipped with hydraulic power steering systems, in which a manual steering wheel is mechanically forcibly coupled with the steerable vehicle wheels. The steerable vehicle wheel is also coupled with respect to the drive to a servo motor which is controlled as a function of the forces and moments transmitted between the manual steering wheel and the steered vehicle wheels, specifically such that the manual force required for the respective steering maneuver on the steering handle is more or less clearly reduced. For controlling the servo motor, as a rule, parts of the mechanical driving connection between the manual steering wheel and the steered vehicle wheels are elastically coupled with one another so that, corresponding to the respective effective forces and moments, the mentioned parts carry out a more or less large stroke or a more or less large rotation relative to one another. With this stroke or by way of this relative rotation, in the case of a hydraulic servo motor, a servo valve is then operated which controls the motor. Optionally, an electric adjusting assembly used as a servo motor can also be controlled by the stroke or the relative rotation.
In airplanes, tail units and landing flaps and the like are connected with assigned handles only with respect to the effect by way of a controlling system. The handle operates a desired value generator and the landing flaps or tail units operates an actual value generator. The controlling device of the controlling system processes the signals of the desired value generator and of the actual value generator in the sense of a desired value--actual value comparison. As a function of the result of this comparison, motor-driven actuating assemblies the tail units or landing flaps are controlled. This concept, which is also called "Fly by Wire", has in the meantime become so reliable that also passenger planes are equipped correspondingly.
Comparable arrangements may basically be provided in the case of vehicle steering systems, as shown, for example, in DE 195 40 956 C1, in which a vehicle steering system is described. A special advantage of such a known arrangement is the fact that, on the one hand, virtually arbitrary steering kinematics, and particularly also an arbitrary transmission ratio between the adjusting movements of the steering handle and the steering movements of the steered vehicle wheels, can be implemented. If equipped with a corresponding sensor system, the system is easily suitable for compensating interference parameters, such as cross wind influences, in a manner to be discussed.
In above-mentioned DE 195 40 956 C1, a mechanical through-drive between the steering handle and the steered vehicle wheels is not completely eliminated. On the contrary, such a through-drive is opened up when the control system operates without fault and thus the through-drive is rendered ineffective. If, however, a malfunction is determined in the control system, which constantly monitors itself with respect to faults, the mechanical through-drive is automatically switched on to become operative, i.e., the mechanical through-drive forms an "emergency level" in the case of a possible malfunctioning of the control system. Furthermore, DE 195 40 1956 C1 describes a non-self-locking electric motor for simulating the manual force.
EP 0 539 823 A1 also shows a vehicle steering system in which the steering handle and the steered vehicle wheels are coupled by way of a controlling system.
DE 32 40 629 C2 describes a conventional vehicle steering system with a continuously effective mechanical through-drive between the manual steering wheel and the steered vehicle wheels in which the steering shaft is constructed with handle-side and steering-transmission-side shaft parts which can be rotated relative to one another against an elastic resistance. The extent and the direction of the relative rotation between these shaft parts allow a signal to be derived for controlling a power assembly. A spring arrangement in the form of a torsion bar or the like may be arranged between the mentioned shaft parts.
In a power steering system described in DE 39 18 987 A1 which has a forced mechanical coupling between the manual steering wheel and the steered vehicle wheels, the required manual force is changed by an electric motor which, in a normal case, generates a parameter-dependent reaction power at the manual steering wheel and, in special cases, can also act as a servo motor.
An object of the present invention is to provide an advantageous embodiment of a steering system in which particularly the controlling of the manual force adjuster is to be improved.
This object has been achieved in a vehicle steering system according to the present invention by providing that the shaft has handle-side and motor-side shaft parts which can be rotated relative to one another against an elastic resistance, and from the extent and the direction of the relative rotation between the shaft parts, an actual value signal can be generated for the manual force.
The present invention is based on the discovery of the general idea of providing between the steering handle and the servo motor acting as a manual force adjuster, of using a mechanical driving connection of a spring-type elasticity and of using the elastic deformations occurring during the operation as an actual value signal of the manual force for controlling the servo motor. This first results in the advantage that the actual values of the manual forces generated by the manual force adjusters can be detected by path generators or angle-of-rotation generators; i.e., the actual value determination of the manual forces can take place in a particularly simple manner with respect to measuring techniques. The advantage is obtained that similar arrangements can be used for the spring-elastic driving connection between the manual force adjuster and the steering handle, as those which were successful for controlling servo motors in conventional power steering systems; i.e., conventional mass-produced products can largely be used.
According to a particularly preferred embodiment of the present invention, it may be provided that, in the event of a malfunctioning of the controlling system between the steering handle and the steered vehicle wheels, a mechanical through-drive becomes effective, which extends via the shaft with the handle-side and motor-side shaft parts, which are rotatable with respect to one another against an elastic resistance, and is opened up during the correct functioning of the controlling system. The servo motor of the manual force adjuster operates as a servo motor which is controlled as a function of the extent and of the direction of the relative rotation of the shaft parts.
The above-mentioned shaft has a double function here in that, during the normal operation when the servo motor is used for generating a manual force, the elasticity of the shaft is used for the actual value determination of the manual force. In emergency situations, when the servo motor is to operate as a booster motor, the elasticity is to be used for controlling the servo assistance.
In this manner, a largely conventional power steering system can be made available in a constructively simple manner as an emergency plane, in which the components used for generating and controlling the servo power are used during the normal steer-by-wire operation for generating or detecting the manual force.